The present invention relates to a support for the lower part of a fused salt electrolytic cell used in the production of aluminum. The cell comprises an electrolyte tank with an outer steel tank, a thermally insulating layer and an electrically conductive inner carbon lining which is able to withstand the molten contents of the cell.
Aluminum is produced from aluminum oxide by the fused salt electrolytic process in which the aluminum oxide is dissolved in a fluoride melt comprised of, for the greater part, cryolite. The cathodically deposited aluminum collects under the fluoride melt on the carbon floor of the cell. The surface of the liquid aluminum provided forms the cathode. Dipping into the melt from above are anodes which, in conventional processes, are made of amorphous carbon. As a result of the electrolytic decomposition of the aluminum oxide, oxygen forms at the carbon anodes and combines with the carbon therein to form CO.sub.2 and CO. The electrolytic process takes place in the temperature range around 940.degree.-970.degree. C.
During the course of production, the carbon lining experiences a significant increase in volume due to the penetration by components from the electrolyte. The term components as used here is to be understood to mean for example, sodium or salts making up the fluoride melt and chemical compounds which arise from unidentified reactions in the fluoride melt.
As a result of the increase in volume, the carbon lining exerts pressure on the thermal insulation and thus indirectly on the steel tank which suffers irreversible plastic deformation capable of producing cracks in the tank. The carbon lining itself becomes deformed, mostly with the floor doming upwards, resulting in cracks therein. The liquid aluminum may then leak through these cracks and attack the iron cathode bars which conduct away the direct electric current. The destruction of the cell lining may advance to such a degree that the molten aluminum runs out of the cell. In such a case the cell usually has to be put out of service. The cracking of the cell leads to expensive repairs as well as production loss due to the downtime of the cell.
Many attempts have been made in the past to prevent such deformation and cracks by reinforcing the steel tank. Such attempts have not been able to prevent the deformation and cracking and have only been able to reduce the frequency of occurrence. Furthermore, attempts to reinforce the cell represent a significant economic handicap as the cell becomes much more expensive. In addition the total weight of the cell tank is greatly increased.
Other attempts at overcoming the problem have aimed at saturating the carbon lining with electrolyte components in an attempt to overcome the increase in volume so produced thereby. It has been found, however, that the increase in volume cannot be avoided and must be accepted as a given characteristic.
In the German patent application published for opposition DE-AS 1 005 739 an attempt is made to increase both the strength and the service life of the steel tank by constructing the tank out of a number of different parts which can be displaced relative to each other. These individual parts are mounted by means of elastic elements on the stationary frame above the cell. However, since the means of reinforcement is replaced by a complicated tank construction the overall investment costs remain high.
In U.S. Pat. No. 4,124,476 it is suggested that a projection be provided in the steel tank. This comprises a first, easily deformable material and a second material which deforms only under higher forces, filling completely a space to accommodate the floor of the carbon lining, which expands in the horizontal direction during operation of the cell. The second material exhibits such properties that the forces can be transformed to the shaped steel shell without permanent deformation and/or forming cracks. The counteractive forces operating on the floor of the carbon lining reduce the degree of doming by the floor and the formation of through cracks there.
A suggestion to fit the electrolyte tank with reinforcements but to permit the elastic expansion of the tank is made in U.S. Pat. No. 4,322,282. The reinforcements on the sidewalls of the cell are elastic and are mounted with facilities which permit them to be moved. These reinforcing elements are preferably hollow so that a temperature gradient of 100.degree.-200.degree. C. can form in the section.
Finally, in German patent application No. 21 22 246 an electrolytic cell is described wherein the steel shell is in the form of a box and is provided with horizontal beams on the short sides and floor of the cell and vertical beams on the long sides. Bolts, with round nuts, provide a hinged connection between the floor and the vertical beams, the lower ends of which are supported in pairs by spacers.
Although the state of the art suggestions provide some alleviation of the difficulties, electrolytic cells with extremely high current densities still produce very great problems. Modern aluminum fused salt electrolytic cells with a power rating of over 200 kA must be constructed longer and not broader because this is the way that magnetic problems can be more easily kept under control. With such long, narrow cells there is a greater tendency for thermal expansion of the carbon lining to cause torsional distortion along the longitudinal axis which can even lead to buckling perpendicular to the long axis of the cell. This so called "shoe-box" effect must therefore be prevented without anchoring the cell in a manner that results in cracks starting to form.
It is therefore a principal object of the present invention to develop a mechanism for the lower part of a fused salt electrolytic cell which in all sizes, in particular cells with power ratings over 200 kA, are capable of preventing uncontrollable deformation of the cell so as to prevent damage due to crack formation. The mechanism is realizable at low investment cost and is flexible in its application.